1. Field of the Invention
The present invention relates to a charging device, and more particularly, to a contact-type charging device employed in an electronic photographic printer. The present invention also relates to a transfer device employed in an electronic photographic printer.
2. Description of the Related Art
An electronic photographic printer comprises a charging device, an exposure device, a developing device, a transfer device, and a fixation device.
Printing of an image onto a recording medium (such as a paper, etc.) is performed using the following procedures:
The charging device electrically charges the surface of a photoconductor to have a predetermined potential. Particularly, the charging device includes a conductive charge roller which is in contact with the surface of the photoconductor. The charging device applies a DC constant voltage to the charge roller, thereby the surface of the photoconductor is charged with a predetermined potential.
The exposure device exposes the photoconductor in a pattern corresponding to a to-be-printed image. In an area of the surface on which a light beam is irradiated, the charges disappear. Thus, an electrostatic latent image of the to-be-printed image is formed on the surface of the photoconductor.
Subsequently, the developing device forms the toner image on the surface of the photoconductor. Particularly, the developing device electrically charges toner as to have a polarity which is opposite to a polarity of the photoconductor, and affixes the toner onto the surface of the photoconductor. Having performed this, the toner image is formed on the surface of the photoconductor.
After this, the transfer device transfers the toner image formed on the surface of the photoconductor onto a recording medium. Particularly, the transfer device includes a conductive transfer roller. A recording medium is sandwiched between the transfer roller and the photoconductor. The transfer device applies a DC constant voltage to the transfer roller, so as to attract the toner image onto the recording medium. Hence, the toner image is transferred from the photoconductor onto the recording medium.
Finally, the fixation device fixes the toner image on the recording medium, thus completing the printing of the image on the recording medium.
The current flowing to the charge roller varies, depending on the temperature, the humidity, and the abrasion of the photoconductor. Thus, if the voltage to be applied to the charge roller is constant, the charged potential of the photoconductor also varies in accordance with the temperature, the humidity and the abrasion of the photoconductor. Such variation in the potential causes deterioration in the printing.
Particularly, if the photoconductor is not sufficiently charged, the toner is not affixed onto the surface of the photoconductor. This results in creating an unclear printing image. On the contrary, if the photoconductor is excessively charged, a large amount of toner is affixed thereonto. This results in making a large mess on the recording medium.
In order to overcome the above problems, according to the technique disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H6-35302, a current flowing to a charge roller is measured, and a DC voltage to be applied to the charge roller is controlled in accordance with the result of the measurement.
In the technique of Unexamined Japanese Patent Application KOKAI Publication No. H6-35302, it is necessary to prepare a current measurement circuit, which measures the current flowing to the charge roller, and a controller, which controls the voltage in accordance with the measured result. Hence, the electronic photographic printer is manufactured at a high cost.
Likewise the above, the current flowing to the transfer roller also varies based on the temperature, the humidity, etc. Thus, if the voltage to be applied to the transfer roller is constant, the charged potential of the transfer roller is not stable. Therefore, the toner image on the photoconductor may not completely be transferred onto the recording medium.
When the current flowing to the transfer roller increases, unless the voltage to be applied to the transfer roller decreases, a leakage current may occur. In this structure, the toner image on the photoconductor can not be transferred, or the toner may be scattered around. When the current flowing to the transfer roller decreases, unless the voltage to be applied to the transfer roller increases, the adequate potential for transferring the toner image can not be obtained. Hence, the deterioration of the printing quality occurs.
In the technique of Unexamined Japanese Patent Application KOKAI Publication No. H6-35302, the above-described problems have not been mentioned. Thus, the above-described problems can not be overcome, even the technique of the publication is employed.
The entire disclosure of the above publication is incorporated herein by reference.
It is accordingly an object of the present invention to provide a charging device and a transfer device which can realize high quality printing at a low cost.
In order to achieve above object, according to the first aspect of the present invention, there is provided a charging device comprising:
a conductor which is in contact with a photoconductor; and
a power source circuit which applies a voltage to the conductor, thereby to electrically charge the photoconductor as to have a predetermined potential, and
wherein the power source circuit includes a resistor having a predetermined level of resistance, and provides the conductor with a voltage through the resistor.
According to this invention, the power source circuit provides the conductor with a voltage through the resistor. Thus, the voltage to be applied to the conductor varies in accordance with the current flowing to the conductor. Particularly, the higher the current flowing to the conductor, the lower the voltage applied to the conductor. On the other hand, the lower the current flowing to the conductor, the higher the voltage applied to the conductor. This can prevent the variation in the charged potential of the photoconductor. In the structure where only the resistor additionally included in the power source circuit, the charged potential of the photoconductor can be stable. Therefore, high quality printing can be realized at a low cost.
The power source circuit may include a rectification circuit which provides a predetermined level of a voltage; and
the resistor may be connected in series between the rectification circuit and the conductor.
Resistance of the resistor may be in a range between 1 and 2 mega-ohms, most preferably 1.5 mega-ohms.
In order to achieve the above object, according to the second aspect of the present invention, there is provided a transfer device comprising:
a conductor which faces through a recording medium, a photoconductor on whose surface a toner image is formed; and
a power source circuit which applies a voltage to the conductor, thereby to transfer the toner image on the recording medium, and
wherein the power source circuit includes a resistor having a predetermined level of resistance, and provides the conductor with a voltage through the resistor.
According to this invention, the power source circuit provides the conductor with a voltage through the resistor. Hence, the voltage to be applied to the conductor varies in accordance with the current flowing to the conductor. Particularly, the higher the current flowing to the conductor, the lower the voltage applied to the conductor. On the other hand, the lower the current flowing to the conductor, the higher the voltage applied to the conductor. This stabilize the potential of the conductor, resulting in providing high quality printing. Only the resistor is additionally included in the power source circuit, realizing such printing at a low price.
The power source circuit may include a rectification circuit which provides a predetermined voltage; and
the resistor may be connected in series between the rectification circuit and the conductor.